I was lucky enough to find the Hogwarts Express set on eBay at a reasonable price. Got it in, and everything runs once I took it to the "local" (60 miles) Marklin dealer and they added weight to the tender which solved the running issues. But, after I've run for less than an hour, the loco has started making a high pitched noise somewhere between a squeak and a grind.

I don't think the dealer oiled it while they were inside it, so I'm thinking that maybe a drop of oil would be the best bet and check for anything that might have gotten in the gears? I should mention that my layout is very much under construction, so it's certainly possible that something got picked up.

Other ideas?